Vehicles having an internal combustion engine can operate in a variety of modes. As one example, an engine may operate in a spark ignition (SI) mode, wherein a charge of a mixture of air and fuel is ignited by a spark performed by a sparking device within a combustion chamber. As another example, an engine may operate in a compression ignition mode, wherein a mixture of air and fuel are compressed within a combustion chamber by a piston, causing ignition of the charge without necessarily requiring the addition of a spark from a sparking device.
One type of compression ignition known as homogeneous charge compression ignition (HCCI) utilizes compression of a substantially homogeneous mixture of air and fuel to achieve controlled autoignition (CAI). In some conditions, HCCI includes the advantages of improved fuel efficiency and/or reduced emissions over other operating modes, such as SI. However, autoignition may not always be reliably achieved, during some conditions. For example, at some engine loads and/or speeds, the engine may not produce enough heat to maintain autoignition, thereby potentially causing misfire, noise and vibration harshness (NVH), and/or decreased engine performance.
In one approach, as described in U.S. Pat. No. 6,135,088, an engine may be controlled to operate in a spark ignition mode or a stratified self-ignition mode by varying an exhaust throttle to control the exhaust gas recirculation (EGR) that is provided to the engine.
However, the inventors herein have recognized a disadvantage with this approach. Specifically, under some conditions, the use of exhaust gas throttling to maintain combustion over an extended duration can cause a decrease in engine efficiency, thereby reducing fuel economy. In other words, this approach may utilize exhaust throttling to such an extent to maintain combustion performance that efficiency gains achieved via compression ignition are overcome by throttling efficiency losses.
In another approach, the above issues may be addressed by a method of operating an engine for a vehicle having at least one cylinder and an exhaust passage communicatively coupled to the cylinder including at least one exhaust throttle disposed therein downstream of the cylinder, the method comprising operating the cylinder to achieve controlled autoignition of an air and a fuel mixture with the cylinder; during a first transient condition, adjusting at least the exhaust throttle to maintain controlled autoignition within the cylinder; during a second transient condition, adjusting at least an actuator other than the exhaust throttle to maintain controlled autoignition within the cylinder; wherein said first transient condition is more rapid than said second transient condition.
In this way, it may be possible to coordinate various control operations of the engine by recognizing that some control operations provide more rapid response than others, while at the same time considering overall system efficiency. For example, the exhaust throttle may be used to respond to more rapid transient conditions such as gear changes to maintain homogeneous charge compression ignition operation, while other control devices such as intake and/or exhaust valves, turbo-charging, intake air conditioning or heating may be adjusted to respond to less rapid transient conditions.